Vertical Beam Instability Interlock System ('Fuse')

General description :

Two fully independent, but identical, vertical instability interlock devices are installed and
operational just behind the dipoles of Cells 5 and 19 respectively. Such a device is generally refered to as the ‘Fuse’.

A 'Fuse' basically consists of a 2 small blades (up & down), vertically separated 2mm (fixed), with three (for reliability redundancy) thermocouples mounted on each of them. The assembly is in vacuum, and aligned on the photon-beam from bending magnet (that is not used for a beamline).

It can be vertically translated to be either completely out (i.e. ‘home’ or non-active) or to be (vertically) centered on the photon-beam (i.e. ‘inserted’ or active). In the latter case the readings of the thermocouples (when of roughly equal value up & down) determine that the 2mm slot is effectively centered.
With the ‘Fuse’ device at 3.2m from the source point, a 2mm gap was chosen in order to have a small amount of power (i.e. a fraction of a Watt at nominal SR current of 200mA) deposited on the blades, which is just enough to determine the center. The blades of the ‘Fuse’ are at right angles to the photon beam and of small dimensions and mass, making them thermally very sensitive to a change of power absorption i.e. a change of vertical beam position and/or size.
In the case of a strong vertical beam blowup, or a rapid vertically oscillating beam position, a significant temperature rise on both blades will be caused. By setting an appropriate interlock temperature level on each of the six independent modules that monitor the thermocouple readings, all the other vacuum parts in the Storage Ring machine that all have a much larger vertical aperture than 2mm are protected against such abnormal beam behaviour or conditions in the vertical plane.